Lightweight is scary! The aggregate is very porous and as such holds a great amount of water. Drying time for lightweight concrete can be 300% longer than standard concrete. I'd figure 3 months per inch of thickness and go from there.

I had a job that was on 11" of lightweight and I had MVER readings of 3lbs - 4lbs, with RH readings of 96%. The floor failed after the first year (that's when I was called in) and we ended up coating the floor with a Moisture Mitigation System.